The aim of this study is the elucidation of the mechanisms involved in the transport of vitamin B12 across the cell envelope of Escherichia coli. Our previous work has shown that this process requires at least three genes, BtuB, BtuC and TonB. The gene BtuB (presumably identical with Bfe) codes for the outer membrane, B12 receptor protein which also functions as the receptor for bacteriophage BF23 to the 12th power and the colicins. In the presence of an active proton motive force and the TonB and BtuC gene products the vitamin B12 bound to the receptor is transported across the inner membrane and into the interior of the cell. Our current primary objective is to identify all of the components that come into contact with vitamin B12 during its passage through the cell envelope. The techniques to be used will include photoaffinity labeling with tritium-labeled corrinoid derivatives of nitrophenylazide followed by analysis of the envelope compounds that become labeled, using polyacrylamide gel electrophoresis for proteins and thin layer chromatography for phospholipids. These photoaffinity labeling experiments will be done with cells under various physiological conditions and with cell strains that are mutant in different parts of the B12 transport process.